Có một số lượng các oxit nitơ, nitơ oxit (N2O), oxit nitric (NO), nitơ đioxit (NO2), nitơ trioxide (N2O3), và pentôxít nitơ (N2O5), được gọi chung là NOx. Hai oxit nitơ được quan tâm hàng đầu đến ô nhiễm không khí là no và NO2. NO là một chất khí không màu, đó là một tiền thân của NO2 và là một hợp chất hoạt động trong các phản ứng quang hóa sản xuất sương khói. NO2 là một chất khí màu nâu đỏ cho màu khói và có thể đóng góp để opacity trong luồng khí thải từ ngăn. | 17 NOx Control There are a number of oxides of nitrogen including nitrous oxide N2O nitric oxide NO nitrogen dioxide NO2 nitrogen trioxide N2O3 and nitrogen pentoxide N2O5 that are referred to collectively as NOx. The two oxides of nitrogen that are of primary concern to air pollution are NO and NO2. NO is a colorless gas that is a precursor to NO2 and is an active compound in photochemical reactions that produce smog. NO2 is a reddish brown gas that gives color to smog and can contribute to opacity in flue gas plumes from stacks. NO2 is a criteria pollutant with a National Ambient Air Quality Standard of 100 pg m3 or .053 ppm annual average. It is also a precursor to nitric acid HNO3 in the atmosphere and is a major contributor to acid rain although less important than SO2 which is discussed in the next chapter. Nitric acid contributes only one proton per molecule while sulfuric acid has two protons per molecule and mass emissions of sulfur compounds are larger than oxides of nitrogen. Finally NOx and volatile organic compounds VOC react photochemically in a complex series of reactions to produce smog which includes ozone NO2 peroxyacetyl nitrate PAN peroxybenzoyl nitrate PBN and other trace oxidizing agents. By far the largest source of NOx is combustion although there are other industrial sources such as nitric acid manufacturing. Figure shows the relative contribution from NOx emission sources. The large amount of NOx generated at coal-fired electric power plants is evident and the very large contribution from motor vehicles and other forms of transportation including ships airplanes and trains is pronounced. Figure shows that total NOx emissions have been fairly steady at about 23 tons per year despite industrial growth and a growing number of vehicles on the road. Preventing an increase in total NOx emissions can be attributed to the increased use of NOx controls especially in automobiles and in industrial fuel consumption. NOX FROM COMBUSTION
